Ground circuit impedance measurement apparatus and method

ABSTRACT

Novel ground circuit impedance measurement apparatus and method accurately measure ground circuit impedance. A ground circuit and electrical current source of 25 amperes or less simulate a ground fault. Ground impedance is determined by dividing voltage drop by amperage to determine impedance directly. In one aspect, impedance is determined and displayed automatically. Power wires transmit voltage and current data for the circuit being measured. The cable itself serves as a current-carrying and voltage-transmitting conduit to determine directly the circuit impedance. The novel ground circuit impedance device and method measure impedance in underground mine power systems connecting to an outside electrical power substation outside the mine and using portable transformers in the mine. In one aspect, ground circuit impedance is measured automatically in cables over 1 mile long.

This application is a Provisional of U.S. patent application U.S. Ser.No. 60/403,825 filed Aug. 15, 2002.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to impedance measurement apparatus and method. Inone aspect, this invention relates to the design, construction, andinstallation of ground circuit impedance measurement apparatus in anunderground coal mine. In one aspect, this invention relates to thedesign, construction, and installation of ground circuit impedancemeasurement apparatus in other industrial applications where accuratemeasurement of ground circuit impedance is required.

2. Background

In underground coal mines, as well as other industrial applications,medium voltage and high voltage electrical power equipment is commonlyused. As an example, in coal mines, the portable, high voltageelectrical power equipment is connected to an outside electrical powersubstation by lengths of high voltage cable with high voltage connectorsto couple the cables together as well as to portable transformers.

In order to insure the safety of mine personnel, federal laws requirethat all high voltage underground equipment be installed with adequategrounding means to insure that the voltage on such equipment not exceed100 volts in the event of a ground fault (30 CFR 75.001).

INTRODUCTION TO THE INVENTION

No existing products are available which measure ground impedance oncertain type circuits. The few companies that do attempt to estimate thecircuit impedance do so by means of calculation which involves wireresistance tables when the wire size and length is known. What thesecalculations fail to take into consideration is the resistance ofnumerous connections such as high voltage couplers and boltedconnections.

The specific problems to be overcome include providing a means ofmeasuring impedance on a circuit which extends as much as several milesin length.

In underground mining applications, the ground fault current is limitedto 25 amperes by a neutral grounding resistor. The total ground circuitimpedance must not exceed 4 ohms in order to stay under the 100 voltsallowed on the equipment frame. Ohm's Law shows the relationship ofvoltage to current and impedance:V=IZV=(25 amps) (4 ohms)

Although the Federal Coal Mine Health and Safety Act of 1969 mandatedthe grounding requirement years ago, there remains to this day nopractical means of accurately determining ground circuit impedance. Thecircuit impedance can be estimated by means of calculations whichinvolve wire resistance tables when the wire size and wire length areknown. These calculations fail to take into consideration the resistanceof numerous connections such as high voltage couplers and boltedconnections.

A product is needed to measure directly, without calculations, totalcircuit impedance including connection impedance.

SUMMARY OF THE INVENTION

The present invention provides novel apparatus and method for measuringground circuit impedance including a ground circuit having an electricalcurrent source, disconnecting electrical power by opening a main circuitbreaker and a visible disconnect switch, a means of connecting powerconductors, to the ground conductor at the end of the circuit,connecting an electrical alternating current source which passes currentthrough the ground circuit, accurately measuring both the currentintroduced into the circuit and the resulting voltage drop produced inthe ground circuit, and automatically calculating the ground impedance.

In one aspect, alligator clips are used to connect a current source toone phase wire and the ground wire. Another alligator clip is used toconnect the voltage sensing device to another phase lead, which measuresthe voltage of that phase lead with respect to the ground wireconnection. Since all phase leads are connected together at the far endof the circuit to be measured, the measures voltage is that whichresults from the current through the ground circuit. In applicationswhere cable couplers are used, a special coupler may be used toconveniently connect all power conductors to the ground conductor. Powerwires in the circuit to be measured serve as a means for carryingcurrent and transmitting voltage data to determine directly the groundcircuit impedance.

In underground mining applications, the novel ground circuit impedancedevice measures impedance in an underground mine where undergroundportable transformers are connected to the outside electrical powersubstation.

In applications which use cable couplers, inserting a special groundingcoupler to provide a connection to ground off all power conductors mayalso be used to provide safety during inspection or troubleshooting.

Ground circuit impedance is measured automatically in cables of anylength, including several miles in length.

Further embodiments of the present invention will become apparent fromthe detailed description which follows in conjunction with the figuresof the drawings.

BRIEF DESCRIPTION THE DRAWINGS

FIG. 1 is a schematic diagram of the high voltage circuit to be testedby the present invention including the ground wire and numerousconnections which connect six high voltage transformer frames to anearth ground bed via a high voltage cable.

FIG. 2 is schematic diagram of a high voltage coupler of the presentinvention.

FIG. 3 is a schematic diagram of an AC current source and monitoringgauges for the apparatus and method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus and method of the present invention provide novel groundcircuit impedance measuring in medium and high voltage electrical powersystems.

Specific embodiments of the apparatus and method of the presentinvention include means for circulating current through a groundingcircuit and measuring the voltage drop across the ground circuit todetermine circuit impedance. The novel device of this invention uses oneof the power wires to carry current, and another power wire as means tomeasure voltage.

In one aspect, the novel apparatus and method of the present inventionprovide a means of measuring circuit impedance on very long cables. Someuses of the present invention provide for direct measurement of eitherground circuit impedance or power circuit impedance.

The apparatus and method of the high voltage coupler component, whenused, of the present invention also include a preferred method of makingthe novel apparatus by inserting the coupler in the end of the cablecoupler, a connection to ground of all three phase wires in order toprovide safety during inspection or troubleshooting.

The present invention also includes a preferred method of installing andconstructing the ground impedance measurement apparatus of the presentinvention.

One of the problems of attempting to measure the impedance of very longcircuits is that the two end connections of the circuit to be measuredare often miles apart.

The apparatus and method of the present invention provide for means andmethod for using the cable itself as a current-carrying andvoltage-transmitting conduit in order to determine directly the circuitimpedance.

To facilitate an understanding of the means and method of operation inaccordance with the present invention, reference is made to theillustrations presented in the figures of the drawings.

FIG. 1 is a schematic diagram of the high voltage circuit to be testedby the present invention including the ground wire and numerousconnections which connect six high voltage transformer frames to anearth ground bed via a high voltage cable.

Referring now to FIG. 1, an example is shown of a typical high voltageelectrical power system, as would commonly be found in an undergroundmine. Substation 10 is associated with neutral grounding resistor 12 andearth ground bed 14. High voltage cable 16 carries current fromsubstation 10 to high voltage coupler 18 and step down transformer 20,22, 23, 24, 25 and, 26, which transform the distribution voltage toutilization voltage at difference locations.

FIG. 2 is schematic diagram of a high voltage coupler of the presentinvention.

Referring now to FIG. 2, a schematic diagram illustrates a high voltagecoupler of the present invention. High voltage cable 28 is connected tohigh voltage complex 30 which connects to all three power wires 32, 34,and 36 to ground wire 38.

FIG. 3 is a schematic diagram of an AC current source and monitoringgauges for the apparatus and method of the present invention. Neutralgrounding resistor is shown at 40. Voltmeter 50, ammeter 52, and ACcurrent source 54 are shown connected to high voltage cable 60. Phase 1is indicated at 62 and phase 2 is indicated at 64. Phase 3 is indicatedat 66, and ground is indicated at 68.

Referring now to FIG. 3, a schematic diagram illustrates an AC currentsource and monitoring gauges for the apparatus and method of the presentinvention.

The present invention provides novel apparatus and method for accuratelymeasuring the ground circuit impedance by completing the followingsimple steps. First, the power is disconnected by opening the maincircuit breaker and visible disconnect switch. Next, a special highvoltage coupler as illustrated in FIG. 2 is inserted into the highvoltage coupler at the end of the circuit. Finally, the AC currentsource with ammeter and voltmeter as illustrated in FIG. 3 is connectedvia alligator clips to two of the phase wires and the ground wire.

The current is adjusted to 25 amperes or less, which simulates a groundfault by passing the current through the ground circuit. The voltmeterreads the total voltage drop across the ground circuit. The groundimpedance, by Ohm's law, is determined by dividing the voltage readingby the current reading.

In respect to a current of 25 amperes, to allow this unit to be morecompact, a smaller current is used, which consumes less power andprovides for easier portability. The original concept does not change inthat the ground impedance is determined by dividing the voltage by thecurrent, but the calculation is made automatically by a microprocessorwhich allows the impedance to be read directly on a meter or digitaldisplay.

The apparatus and method of the present invention also are useful inlocating poor connections in high voltage couplers used in undergroundand surface mines. Poor connections result in a higher than normalresistance circuit, which connections are detected by the apparatus andmethod of the present invention.

The method and procedure of the present invention used in measuringground impedance are used for locating poor connections, but rather thanconnecting one of the alligator clip leads to the ground wire, all threeof the leads from the AC current source are connected to the phaseleads.

Several advantages are obtained through the use of the preferredapparatus and method of the present invention disclosed in the detaileddescription herein below. One advantage is an ability to determineaccurately if the total ground circuit impedance rises to an unsafelevel, where personnel may be exposed to unsafe voltage levels onequipment frames during ground fault conditions.

A further advantage of the apparatus and method of the present inventionis an ability to detect poor connections in high voltage couplers which,if left undetected, could result in coupler failure due to overheating.

To facilitate an understanding of the characteristics of the invention,an underground coal mine application is used as an example.

Installation of the invention in an underground coal mine setting orelsewhere is performed by an MSHA-certified electrician who would insurethat the circuit is positively disconnected before attaching the highvoltage shorting coupler to the end of the cable.

The apparatus and method of the operation of the present inventionprovide the following differences over previously available impedancemeasurement devices.

Key benefits and features include the ability to accurately measureimpedance directly on extremely long circuits by using the cable itselfas a means of transmitting current and voltage data.

The measurement system of the present invention provides an effectiveand economical means to accurately determine the true impedance of ahigh voltage electrical power circuit or a high voltage ground circuit.

The high voltage coupler connects the three power conductors to groundat the end of the circuit being tested. This component has somenoteworthy advantages over anything presently available for use inshorting power wires to ground for safety reasons during maintenance,trouble-shooting, or inspection.

Returning to the overall system, it has been found that no device ispresently available which will perform the function this device does.The problem with measuring impedance on these high voltage circuitsresults from the very long distance which separates the two ends of thecircuit. If the high voltage cable were “looped back” so that both endsof the cable were next to each other, measuring the ground circuitimpedance by an ohmmeter connected to each end of the cable wouldprovide an accurate measurement. When the cable ends are far apart,measurement with an ohmmeter requires one lead of the meter be as longas the cable which is impractical and the measurement includes theimpedance of the meter lead.

The device of the present invention uses the cable itself as a meterlead to measure the voltage which results across the ground circuit whena current is applied through it. Using the novel device, the impedanceof the cable used to measure, acting as a “meter lead” has no influenceon the accuracy of the measurement.

In respect to the words “impedance” and “resistance,” the two words aresynonymous in that they both describe an opposition to the flow ofelectrical current, but “impedance” is used in alternating current (AC)circuits, and “resistance” is used in direct current (DC) circuits.Since the novel device has an application in AC circuits only, thecorrect term is “impedance.”

The apparatus and method of the present invention provides specificallyfor measuring ground circuit impedance in high voltage underground minepower systems, but is also useful in many other applications. Theapparatus and method of the present invention provide accurate means ofdetermining the true impedance of the ground circuit.

The apparatus and method of the present invention is light-weight andportable, and powered by rechargeable battery. The apparatus and methodof the present invention measure impedance not just resistance. In oneaspect, one pushbutton operation provides a digital display. Theapparatus and method of the present invention has been found to beaccurate to 0.1 ohm between 0 ohm and 10 ohms.

In applications such as resistance grounded systems where regulationspermit a maximum voltage on equipment frames under ground faultconditions, it is essential the ground circuit impedance be available tooperating personnel. The apparatus and method of the present inventionaccurately measures ground circuit impedance using novel apparatus andmethod which is not affected by pilot wire resistance, contactresistance, or other factors affecting conventional methods.

The present invention has been found to be useful in locating poorconnections in high voltage couplers used in underground and surfacemines. Poor connections result in higher than normal resistance circuit,which can be easily detected by this device.

The same procedure used in measuring ground impedance is used, butrather than connecting one of the alligator clip leads to the groundwire, all three of the leads from the AC current source would beconnected to the phase leads.

The apparatus and method of the present invention as shown andillustrated in the detailed description of the specification and theaccompanying figures of the drawings are not intended to be limited tothe specific examples shown and described, but the apparatus and methodof the present invention are intended to include the novel developmentsencompassed by the claims which follow and equivalents thereof.

1. A method for measuring ground circuit impedance, comprising: a)providing a ground circuit having an electrical AC current source forsupplying electrical power to the ground circuit; b) disconnecting theelectrical power by opening a main circuit breaker and a visibledisconnect switch; c) inserting a high voltage coupler at the end of thecircuit; d) connecting the electrical AC current source with an ammeterand a voltmeter; e) providing and adjusting AC current to 25 amperes orless, thereby simulating a ground fault by passing the electrical ACcurrent through the ground circuit; f) measuring total AC voltage dropacross the ground circuit; and g) determining ground impedance bydividing the total AC voltage drop by the electrical AC current todetermine impedance directly.
 2. The method for measuring ground circuitimpedance of claim 1, wherein the ammeter and the voltmeter areconnected by alligator clips to two phase wires and to a ground wire. 3.The method for measuring ground circuit impedance of claim 1, whereinthe impedance is determined and displayed automatically.
 4. The methodfor measuring ground circuit impedance of claim 1, wherein a highvoltage coupler is used to connect the power wires to the ground wire.5. The method for measuring ground circuit impedance of claim 1, whereinthe power wires of the circuit being measured serve as a means fortransmitting voltage and current data.
 6. A method for measuring aground circuit impedance of claim 1, comprising measuring impedance inan underground mine.
 7. A method for measuring ground circuit impedanceof claim 1, wherein said electrical current source comprises an outsideelectrical power substation outside an underground mine.
 8. A method formeasuring ground circuit impedance of claim 1, comprising inserting thecoupler in the end of the cable coupler to provide a connection toground of all three phase wires to provide safety during inspection ortroubleshooting.
 9. A method for measuring ground circuit impedance ofclaim 1, comprising using the cable itself as a current-carrying andvoltage-transmitting conduit to determine directly the circuitimpedance.
 10. A method for measuring ground circuit impedance of claim1, comprising using a portable transformer.
 11. A method for measuringground circuit impedance of claim 1, comprising measuring impedance in acable over 1 mile long.
 12. A ground circuit impedance measurementdevice, comprising: means for circulating AC current through a groundcircuit and means for measuring an AC voltage drop across the groundcircuit to determine circuit impedance, further comprising: a groundcircuit having an electrical AC current source for providing electricalpower to the around circuit; a main circuit breaker and a visibledisconnect switch; a high voltage coupler at the end of the circuit; anammeter and voltmeter; means for adjusting AC current to 25 amperes orless, thereby simulating a around fault by passing the AC currentthrough the ground circuit; means for measuring total AC voltage dropacross the ground circuit; and means for determining around impedance bydividing the AC voltage drop by the AC current reading to determineimpedance directly.
 13. A ground circuit impedance measurement device asset forth in claim 12, comprising one power wire to carry current and asecond power wire to measure voltage.
 14. A ground circuit impedancemeasurement device as set forth in claim 12, comprising means formeasuring circuit impedance on cables longer than one mile.
 15. A groundcircuit impedance measurement device as set forth in claim 12,comprising means for measuring circuit impedance on cables longer thantwo miles.
 16. A ground circuit impedance measurement device as setforth in claim 12, comprising means for measuring circuit impedance oncables longer than three miles.
 17. A ground circuit impedancemeasurement device as set forth in claim 12, comprising cables longerthan two miles wherein the ammeter and voltmeter are connected byalligator clips to two phase wires and to a ground wire.
 18. A groundcircuit impedance measurement device as set forth in claim 12,comprising cables longer than three miles wherein the impedance iscalculated and displayed automatically.
 19. A method of measuring groundimpedance by dividing an indicated AC voltage by an indicated AC currenton cables longer than one mile, comprising: a) providing a groundcircuit in an underground coal mine having an electrical AC currentsource for providing electrical power to the ground circuit; b)disconnecting power by opening a main circuit breaker and a visibledisconnect switch; c) inserting a high voltage coupler at the end of thecircuit; d) connecting the electrical AC current source with an ammeterand voltmeter by alligator clips to two phase wires and to a groundwire; e) providing and adjusting electrical AC current to 25 amperes orless, thereby simulating a ground fault by passing the electrical ACcurrent through the ground circuit; f) measuring total AC voltage dropacross the ground circuit wherein the power wires of the circuit beingmeasured serve as a means of transmitting voltage and current data; andg) determining ground impedance by dividing the AC voltage drop by theAC current reading to determine impedance directly and displayedautomatically.